1. Field of the Invention
The invention relates to a high-temperature shift line switching control in which a shift line for automatic shift control is shifted to a lower speed side when the hydraulic fluid temperature of an automatic transmission is high in a vehicle equipped with the automatic transmission.
2. Description of the Related Art
A known shift controller of a vehicle automatic transmission includes a shift control unit and a shift line map switching control unit. The shift control unit automatically shifts gears of the automatic transmission on the basis of a vehicle state by referring to a prestored basic shift line map. When the hydraulic fluid temperature in the automatic transmission is higher than a predetermined fluid temperature determination value, the shift line map switching control unit switches from the basic shift line map to a high fluid temperature shift line map that is set on a lower speed side with respect to the basic shift line map. Japanese Patent Application Publication No. 2003-207038 (JP-A-2003-207038), for example, describes such a shift controller of a vehicle automatic transmission.
According to the above shift controller, when the hydraulic fluid temperature in the automatic transmission is higher than a predetermined fluid temperature determination value, the shift line map switching control unit switches from the basic shift line map to the high fluid temperature shift line map that is set on a lower vehicle speed side with respect to the basic shift line map. This promotes the automatic transmission to upshift to decrease the rotational speed of an input rotating member, resulting in a decrease in fluid temperature.
However, in the above existing shift controller of the vehicle automatic transmission, when, for example, the shift line map switching control unit switches from the basic shift line map to the high fluid temperature shift line map and, as a result, the hydraulic fluid temperature in the automatic transmission is decreased, the shift line map switching control unit switches from the high fluid temperature shift line map back to the basic shift line map. In that case, there is inconvenience that the automatic transmission unexpectedly downshifts because of the switching of the shift line map to make a driver feel uncomfortable. That is, a downshift line for determining whether to downshift between predetermined gears in the basic shift line map and an upshift line for determining whether to upshift between those predetermined gears in the high fluid temperature shift line map intersect with each other or are adjacent to each other. Thus, when the vehicle state falls within a higher speed side region with respect to the high fluid temperature shift line because of switching back to the basic shift line map, there is a case where a point that indicates a vehicle running state represented by a vehicle speed and a required power related value relatively crosses over a downshift line toward a lower vehicle speed side in the shift line map and, therefore, the automatic transmission unexpectedly downshifts.